Lithographic and flexographic photopolymer printing plates may be formed by curing (hardening) a photocurable compound using light.
The image may be formed in a manner known as computer to film (CTF) which utilises the interposition of a masking element (e.g. a photographic negative) between the photopolymer and the light source, the mask preventing light from reaching the parts of the photopolymer where no ink transference in the finished plate is intended. After exposure, the uncured material is then typically removed by washing, e.g. in a solvent. This process involves the creation of a separate masking element, which is costly, time consuming and an impairment to image quality.
Alternatively it may be formed in a manner known as computer to plate (CTP) also referred to as digital printing; here the image may be formed by fixing the wanted parts using energy from a laser or alternatively removing unwanted parts using laser energy. The printing industry is currently moving towards digital printing to increase production speed, quality of reproduction and ease of use.
The process of forming the image by scanning the resin with laser light results in the exposure of any area of resin occurring in a very short time, for example a fraction of a second. The result can be insufficient polymerisation and, therefore, a poor quality printed image.
Photopolymer used to make flexographic printing plates may be supplied in the form of a solid sheet, in liquid form which is poured and levelled at its point of use, or by use of a pre-made sachet containing liquid polymer. Lithographic photopolymer may be supplied as a thin photo or heat sensitive coating on a suitable backing substrate such as aluminium sheet.
Typical liquid photopolymers which may be used to make flexographic polymer printing plates include, for example, unsaturated polyurethane resins, e.g. polyether urethane polymers, or polyether polyester urethane copolymers such as, for example, polyether polyester urethane methacrylate polymers. Suitable cured polymers for use in making flexographic printing plates for packaging applications have a Shore A durometer hardness of between 20 and 35, those for making hand stamps more typically have a Shore A durometer hardness of between 40 and 58 and those for making embossing or moulding plates have a Shore A durometer hardness of 90. Polymers having characteristics described in this paragraph and the following paragraphs may be used in the invention described below.
Typical lithographic photopolymer plates contain formulations containing a polymerizable monomer, a binder, a photoinitiator and a sensitizing dye. In conventional, ‘wet’ lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In ‘driographic’ printing, the lithographic image consists of ink-accepting and ink-repelling areas and during driographic printing, only ink is supplied to the master.
The photopolymerisable compound in either flexographic or lithographic printing will in commercial practice include a photoinitiator in the known way. Such conventional photoinitiators are typically organic carbonyl compounds. Typically these organic carbonyl compounds are activated outside the daylight part of the spectrum.
Recent developments in photopolymerisable compounds have led to changes in the photoinitiators utilised in starting the polymerisation process by bringing their activity region into the visible part of the spectrum.
These developments have advantages in the manufacture of lithographic plates in that they enable the use of commonly available, low cost and low energy use blue light DVD lasers. EP-A-1048982 and EP-A-1070990 disclose the use of a titanocene photoinitiator in combination with a visible light absorbing dye to enable the use of lasers emitting light in the region of 350 to 450 nm.
International patent application WO 2006/079788 describes a liquid photopolymer preparation, usable in the invention of this specification, adapted to cure under ambient light. The photopolymer here may include a photoinitiator that is activated at wavelengths of over 400 nm. These liquid photopolymer preparations contain photoinitiators such as aromatic phosphine oxides such as bis-acylphosphineoxide, or more preferably metallocene compounds such as titanocene. The entire disclosure of WO 2006/079788 in relation to photopolymer preparations is included herein by reference.
Having regard to the shortcomings of CTP technology, for example the use of laser scanning to form an image on resin, it would be desirable to provide an alternative maskless technology for imagewise curing of photopolymer.